Neutralization
Generation of electrostatic charge on webs (e.g., polymeric webs) occurs frequently in web handling operations, where the web moves over and around various rollers, bars, and other web handling equipment. Electrostatic charge on webs arises from many causes, including the contact and separation of the web from the various rolls and equipment, unwinding/winding rolls of film and exposure of the web to E-beam or corona treatment (AC or DC). Charge in/on the web may also be present from previous processes, such as electrostatic pinning of the film during casting. Electrostatic charges on a web can be detrimental in the area of precision coating, not only because of spark ignition hazards, but also because these electrostatic charges can cause a subsequently coated liquid layer to be disrupted and form undesirable patterns (see, for example, “Coating & Drying Defects”, Gutoff and Cohen, Wiley, NY, 1995). In addition to inhomogeneous charge patterns, homogeneous charge can also generate coating defects.
In the photographic industry, for example, a significant non-uniform thickness distribution of a photographic coating material often results when such material is applied to a randomly charged web. Because of the high surface resistivity of high dielectric materials, such as polyester based materials and the like, used in photographic film, it is fairly common to have relatively high electrostatic charge, of varying intensity and polarity, occupying web areas closely adjacent one another. The use of such coating materials as a component of a photographic positive or negative, for example, often requires the use of relatively thick coatings to provide at least a minimum thickness coating throughout the web and thereby compensate for such non-uniform thickness distribution which necessarily results in an increase in the use of relatively costly photographic coating materials in order to produce an effective coating thickness. Visual effects such as photographic mottle are also a consequence of coating non-uniformly charged webs with photographic coating materials. Past practices included either tolerating this non-uniform charge distribution and its disadvantages or attempting to neutralize a randomly charged web as much as possible prior to applying the photographic coating materials.
Various techniques for neutralizing charged webs are known.
A technique described in U.S. Pat. No. 2,952,559 involves passing a charged web between a pair of opposed grounded pressure rollers that are spring-force biased against opposite web surfaces for the purpose of neutralizing bounded or polarization-type electrostatic charges and then blowing ionized air onto surfaces of the web to first neutralize surface charges and then establish a particular web surface charge level prior to coating same. This resulting surface charge level is compensated for by applying a voltage to the coating applicator during the actual coating process having a polarity that is opposite to that of the web surface charge.
Another technique, described in U.S. Pat. No. 3,730,753, involves “flooding” a web surface with charged particles of a first polarity so as to generally uniformly charge the surface and thereafter removing the charge imparted to said web surface so as to leave the surface generally free of charge. The amount of charge added to and/or the amount of charge removed from the web surface may be so controlled that the charge variation and the net charge on the surface is lowered to an acceptable level.
In addition to the methods referenced above, there are also commercially-available neutralization systems, such as:
Air ionizers, which provide a source of ionized air. Air naturally contains ions. However, these ions are not sufficiently abundant in most cases to neutralize static charges rapidly enough to protect static sensitive devices. Further, air ions are removed by HEPA and ULPA filters in clean rooms.
Electrical Static Eliminators, which consist of one or more electrodes and a high voltage power supply. Ion generation from electrical static eliminators occurs in the air space surrounding the high voltage electrodes. These ions are then attracted to the static charge on the material, resulting in neutralization. There are various commercial sources for electrical static eliminators, such as MKS Ion Systems and Simco (an Illinois Tool Works company).
Induction Static Eliminators, which are passive devices where neutralizing ions are generated in response to the electric field due to the static charge on the material. Examples of common induction static eliminators include STATIC STRING™, tinsel, needle bars, and brushes.
Nuclear Static Eliminators, which create ions by the irradiation of air molecules. Most models use an alpha particle emitting isotope to create ion pairs to neutralize static charges. These are often also called Nuclear Bars.
Each of these commercially-available neutralization systems provide a means to attain a web that is net neutralized (i.e. such that the magnitude of electric field, as measured with a common static meter, is substantially lower than it was initially, provided the initial charge was substantial). However, the net neutralized web may still have substantial charge.
The use of liquids to neutralize static charge on dielectrics has also been mentioned. The basic idea of neutralizing charge on dielectric materials by exposing the material to at least weakly conductive fluids with a path to ground has been mentioned in the literature (see, for example, page 956 of J. Lowell and A. C. Rose-Innes, Advances in Physics, 1980, Vol. 29, No. 6, 947-1023). For example, U.S. Pat. No. 6,176,245 B1 describes a web cleaning and destaticizing apparatus which removes cleaning solution at a front slot and supplies an undercoat from a back slot. The undercoat is applied in particular to eliminate static generated by the scraping off of the cleaning solution at the front slot. U.S. Pat. No. 6,176,245 B1 places no explicit requirements the electrical conductivity of the destaticizing undercoat, although the example given in U.S. Pat. No. 6,176,245 B1 described a solution containing 88% methyl ethyl ketone, a weakly conductive solution. Also, U.S. Pat. No. 6,176,245 B1 does not explicitly state that the liquid must provide a path to ground, although it is likely that the slotted web cleaning and destaticizing apparatus used in their experiment was made of a conductive material such as a metal. The apparatus is limited to treatment of the same side of the web from which the cleaning solution was removed. There is no discussion regarding the type of charge distributions which would be remediated using the apparatus.
U.S. Pat. No. 6,231,679B1 describes a process using a similar apparatus as described in U.S. Pat. No. 6,176,245 B1. As with U.S. Pat. No. 6,176,245 B1, fluid conductivity or ground path requirements are not discussed. There is no discussion regarding the type of charge distributions which would be remediated using the apparatus.
An older patent, U.S. Pat. No. 2,967,119, describes an ultrasonic process and apparatus that may be used to ultrasonically clean and nonevaporatively dry (e.g. air knifing off the remaining fluid) a continuous film. A purpose of U.S. Pat. No. 2,967,119 is to clean the film, but U.S. Pat. No. 2,967,119 teaches that a further feature of the drier operation is that the film leaves the dryer free of electrostatic charge. This decharging effect is added in several claims, always in conjunction with the nonevaporative drying step. No insight as to the necessary level of fluid conductivity is given in U.S. Pat. No. 2,967,119, and no data is offered that conclusively demonstrates that the destaticizing actually occurs in the drier, rather than in the ultrasonic tank. Furthermore, U.S. Pat. No. 2,967,119 does not specify the types of charge distributions that are addressed by the process and apparatus.
U.S. Pat. Nos. 6,176,245 B1, 6,231,679B1, and 2,967,119 describe the use of liquids to achieve neutralization, but are not directed to dual-side or bipolar charge distributions.
Commercially-available methods for elimination of nontrivial static charge distributions. These charge distributions can cause significant defects in final products.
Generation of a Patterned Charge Distribution on Dielectric Surface:
Charge patterns on a substrate can be used for controlled deposition of material to the charge pattern. The “xerox” method is a familiar example of this process. In the xerox method a photoconductor cylinder is uniformly charged. A light is then used to discharge areas of the photoconductor, leaving an electrostatic pattern. Toner particles are then preferentially attracted to the charged regions on the photoconductor, creating a toner pattern on the photoconductor cylinder. The toner pattern is then transferred to another substrate (such as paper) and fused to set the image on the finished product. There are variations on the xerox method which have been applied to copy machines and laser printers. However, these traditional xerography methods rely on photoconductors which are prone to charge diffusion (line blurring) and decay, and are not able to be charge-patterned robustly on the micrometer length scale and below.
In an attempt to circumvent the limitations of photoconductors, methods have been developed to generate micro- and nano-charge patterns directly on the substrate. These fine charge patterns can then be used to guide deposition of particles to generate micro- or nano-scale features on the substrates. For example, Heiko Jacobs' group from the University of Minnesota has a series of publications (C. R. Barry, J. Gu, and H. O. Jacobs, Nano Letters 5 (10) (2005) 2078; H. O. Jacobs and C. Barry, Patent Application US20050123687(A1)) in which they use “nano-xerography” to create fine charge patterns on an electret substrate to which silver nanoparticles are deposited. In that work, the charge patterns are achieved by direct contact of a charged tool. The tool was created on silicone using lithography and made conductive by plating with gold. The authors claim that silicon stamp features as small as 10 nm can be created which would allow sub-100 micron patterning capability.
All xerography methods, including “micro-xerography” and “nano-xerography”, rely on the ability to generate controlled charge patterns on a substrate. Reported methods of generating charge patterns on the micro- and nano-scale through direct contact charging include the use of atomic force microscopy probes (P. Mesquida, A. Stemmer, Adv. Mater. 13 (18) (2001) 1395; N. Naujoks, A. Stemmer, Microelectronic Engineering 78-79 (2005) 331), stainless steel needles (T. J. Krinke et al, App. Phys. Letters 78 (2001) 3708) or nano-stamps (C. R. Barry, N. Z. Lwin, W. Zheng, and H. O. Jacobs, App. Phys. Letters, 83 (26) (2003) 5527). In addition to these direct contact methods, micro- or nano-scale charge patterns have also been generated using focused ion and electron beams (H. Fudouzi et al, Langmuir 18 (2002) 7648).
The methods of generating controlled charge patterns mentioned above have been able to address the feature size limitations of standard xerography techniques which relied on the charging and discharging photoconductor material. However, the methods mentioned above are generally very slow and/or require the use of special substrates (electrets, for example) to achieve the fine, sharp features demonstrated in the literature.
Another challenge in the area of nano- and micro-xerography is adherence of the final pattern to the substrate. The background mentioned above provides a method of placing charge patterns on dielectric (or electret) substrates which can then be used to guide deposition of a second material. Once the second material (i.e. nanoparticles) is deposited, the issue of adherence must be addressed. For example, this may be done using heat and/or pressure.